The invention concerns a device and a method for calibrating coordinate systems of imaging systems with a tracking system prior to or during image data acquisition, e.g. by means of magnetic resonance tomography.
Modern medical or biomedical imaging methods provide detailed images of living organisms. Such methods include i.a. magnetic resonance tomography MRT, computer tomography CT and nuclear medical imaging, such as e.g. positron emission tomography (PET), single photon emitted computer tomography (SPECT) or intensity-modulated radiation therapy (IMRT).
A large number of these methods require long measurement times of many seconds or minutes and are therefore prone to image quality losses due to movement of the measurement object, a patient or an animal, which could render the recordings useless in that so-called “blurred recordings” are produced.
Methods for reducing or eliminating these aberrations caused by movement of the object are e.g. prospective or retrospective motion correction, in which the movement of the measurement object is either already compensated for during recording by means of suitable tracking of the measuring field or during image reconstruction. For this reason, even in case of movement during the measurement, the generated images do not show any corresponding aberrations, as described in citations 1-11.
The movement information may thereby either be measured by means of the imaging modality itself, often referred to e.g. as “navigators”. However, in most cases this requires an extension or disturbance of the measurement.
Alternatively, the movement information may e.g. be optically detected by a position measurement system or also a tracking system, thereby performing the imaging method itself without disturbance. One necessary step in case of position detection using a system of this type is the calibration of the coordinate systems between the imaging system and the position measurement system, also called tracking system. In this case, the spatial transformation (translation and rotation with a total of six degrees of freedom) between the two modalities must be exactly determined as described in citations 5 and 12.
EP 1 913 333 B1 discloses a device and a method for detecting a drift in calibrated tracking systems for medical applications in order to be able to localize features with respect to one or more coordinate systems within a reference coordinate system using a calibrated tracking system to which a motion sensor is mounted for detecting movements of the tracking system. When such a movement is detected, the tracking system is re-calibrated.
The common methods for calibrating the coordinate systems of tracking systems and imaging systems include measurement of a known object, which is visible for both methods, by both systems, both the imaging modality and the tracking system, either in only one position or in different object positions. The calibration accuracy that can be achieved in this connection depends on the measurement accuracy of both systems and the number of measurements. Due to the fact that one system and in this case usually the imaging system, shows the greater measurement errors and is substantially slower, the accuracy of the procedure is limited and calibration may be time-consuming (minutes to hours).
The problems of these common methods are therefore:
1) For high accuracy, the measurements must be performed in several positions with high measurement accuracy, i.e. with respect to the spatial resolution of the imaging modality, and therefore require long measurement times.
2) The calibration accuracy is limited by measurement errors of both systems e.g. by errors in the reproduction accuracy of the imaging modality.
3) Calibration may be very time-consuming and may take up to several hours.
4) When the systems are changed, re-calibration requires renewed performance of all measurements with the full expenditure of time. Undesired calibration changes may also result from slow instabilities, so-called drifts, in the measurement system.
5) The transfer of the calibration between different imaging modalities and therapy modalities e.g. for radiation therapy, again requires the full calibration effort for each imaging modality.
In view of the above, it is the object of the present invention to obtain fast and precise calibration of the coordinate systems of imaging systems and tracking systems without having to measure an object, which can be detected by both systems, in one or more positions.
A further object of the present invention consists in avoiding useless recordings due to object movements during imaging. It would furthermore be desirable to enable combination of several imaging modalities for detecting objects without increasing the calibration expense.